Soluble covering for cardiac pacing electrode

ABSTRACT

A biocompatible covering, soluble in body fluids, surrounds the fixation helix of an implantable cardiac electrode as the electrode, and its adjoining lead, are inserted intravenously to a selected cardiac chamber. The covering size and shape are selected for protection of blood vessels and other body tissues during insertion, and for exposure of the fixation element shortly after its proper positioning. The covering may be applied to the fixation element by a dip coating process, or formed separately by casting or injection molding, for later attachment to the lead distal end using an adhesive.

BACKGROUND OF THE INVENTION

This invention relates to cardiac diagnostic and chronic therapeuticleads, and more particularly to fixation leads in which an electrodeincludes an anchoring element.

The utility of cardiac pacing leads is well recognized, both forcarrying pulse stimulation signals to the heart from a pacemaker, andfor monitoring heart electrical activity from outside the body. Manysuch leads are sufficiently flexible and small in diameter forintravenous introduction to a cardiac cavity, whereupon an electrode atthe distal end of the lead is implanted into the endocardium to securethe lead. For this purpose, helical coils, barbs and other anchoringelements are provided, typically as part of the electrode.

The anchoring element must be sufficiently sharp to penetrate theendocardium and secure the electrode against becoming detached, forexample due to contractions of the myocardium. During a critical periodimmediately after implant and prior to full fibrotic growth, usuallythree to twelve weeks, the anchoring element must provide substantiallythe entire force maintaining the electrode in its selected location.Given these requirements, it is not surprising that an effectiveanchoring element can become entangled in the vein, heart valve or othertissue encountered during its intravenous insertion.

The problem has given rise to numerous proposed solutions. For example,U.S. Pat. No. 3,974,834 to Kane granted Aug. 17, 1976 shows a sleevewhich shrouds the sharp tip of a fixation helix, but collapses inaccordion-like fashion as the helix is turned into the endocardium. InU.S. Pat. No. 4,282,885 to Bisping granted Aug. 11, 1981, a protectivecore is surrounded by the helix, and is movable axially relative to thehelix. A wire, attached to the core, extends through the lead and can bepulled after lead insertion to withdraw the core, exposing the helix.U.S. Pat. No. 4,146,036 to Dutcher et al granted Mar. 27, 1975 disclosesan extensible and retractable core surrounded by the helix.

Other solutions involve making the fixation element movable. Forexample, in U.S. Pat. No. 4,180,080 to Murphy granted Dec. 25, 1979, aspiral coil, normally recessed within a guide tube, can be rotatedwhereby it emerges beyond the tube. U.S. Pat. No. 3,844,292 to Bolducgranted Oct. 29, 1974 discloses a plunger outside of the body which,after release of two locking mechanisms, is movable to push outward abarb-like tip. A somewhat similar arrangement, involving a platinumpiston movable to push a harpoon-shaped anchor beyond the end of atubular electrode, is shown in U.S. Pat. No. 4,258,724 to Balat et algranted Mar. 31, 1981.

Such devices, while satisfactory in certain respects, are undesirable inthat leads employing them must have a larger diameter. They oftenrequire additional tools, for example a stylet-type screw driver forrotating the helix. Further, such devices are often overly complex,diminishing their reliability and raising the possibility of a currentleakage path between conductors of bipolar leads.

Therefore, it is an object of the present invention to provide a smooth,rounded covering for the anchoring element of a cardiac endocardialelectrode to facilitate intravenous insertion of the electrode.

Another object of the invention is to provide such a covering which issoluble in body fluids, thereby to expose the anchoring element at aspecified time after its initial insertion into the body.

Yet another object is to provide a simple, non-mechanical means forcovering fixation mechanisms during intravenous insertion of a pacingelectrode having an anchoring element, without requiring anylongitudinal relative movement between the electrode and anchoringelement.

SUMMARY OF THE INVENTION

To achieve these and other objects, there is provided an intravascularlead implantable inside a patient's body. The lead includes an electrodehaving a fixation element for effecting penetration into endocardialtissue at a selected location to secure the electrode at the selectedlocation. The lead includes one or more flexible electrical conductors,and one or more flexible, biocompatible dielectric sheaths surroundingthe conductors along substantially their entire length. A coupling meanselectrically and mechanically joins the electrode to a distal end of theconductor, whereby the conductor and electrode transmit electricalsignals from the selected location to the lead proximal end. Abiocompatible covering surrounds the fixation element and facilitatesintravascular movement of the electrode. The covering is soluble inbodily fluids and has a thickness selected to allow at least apredetermined minimum time for the intravascular insertion of the leadand electrode, and for the positioning of the electrode at leastproximate to the selected location, before the covering dissolvessufficiently to expose the fixation element and permit the penetration.

Mannitol, and other sugar derivatives, have been found suitable forforming the covering, which can be produced by dipping the fixationelement into a beaker containing the mannitol or other coveringconstituent at a temperature slightly above its melting point. Thefixation element is removed, cooling the element, along with a portionof material adhering to it. Alternatively, the covering can be preformedas a capsule, with a bore formed in the capsule for accommodating thefixation element. An adhesive is then used to join the covering to theelectrode, with the fixation element inside the bore.

Another aspect of the present invention is an apparatus for facilitatingintravascular insertion of a cardiac pacing electrode. The apparatusincludes a biocompatible, non-pyrogenic covering substantiallysurrounding a fixation element of an electrode. The covering is solublein bodily fluids and has a thickness selected to allow at least apredetermined time for intravascular insertion of the electrode at leastproximate a selected location inside the body of the patient, before thecovering dissolves sufficiently to expose the fixation element to permitpenetration of the fixation element into body tissue at the selectedlocation.

As another aspect of the invention, there is disclosed a process forcoating a fixation element of a body implantable electrode, includingthe steps of:

(a) selecting a biocompatible, non-pyrogenic material soluble in bodilyfluids and having a melting point substantially above normal bodytemperature, and heating the material to a temperature slightly aboveits melting point;

(b) dipping a fixation element of a body implantable electrode into asolution of the material maintained at said temperature;

(c) removing the fixation element, along with an initial portion of thematerial adhering to the fixation element, from the solution andpermitting them to cool a sufficient time for said initial portion to atleast partially solidify;

(d) dipping said fixation element and initial portion into the solutionfor a time sufficient to permit a subsequent portion of the material toadhere to the initial portion; and

(e) removing the fixation element, initial portion and subsequentportion from the solution, and permitting them to cool to an ambienttemperature.

Steps (d) and (e) may be repeated until the thickness of the covering issufficient for the desired dissolving time.

A covering in accordance with the present invention, whether preformedor applied through dip coating, forms a smooth, blunt distal tip for itsassociated lead, allowing an expeditious, intravenous insertion of thelead, without concern that the fixation element will snag upon, tear orotherwise damage the vein or any other tissue as it travels toward theheart. A short time after the electrode reaches the selected cardiacchamber, there is a sufficient dissolving of the covering such that thefixation element is exposed and ready to penetrate the endocardium.

Due to the many materials suitable for the covering, which includevarious salts and polyvinylpyrrolidone as well as the aforementionedsugar derivatives, and further due to controlling the coveringthickness, a wide range of dissolving times is available, so that aparticular covering can be tailored to the expected time for aparticular procedure. Further refinement is provided by the preformedcapsule, due to enhanced control over size, thickness and surface areaof the covering.

IN THE DRAWINGS

For a better appreciation of these and other features and advantages,reference is made to the following detailed description and thedrawings, in which:

FIG. 1 is a side sectional view of the distal end region of animplantable, positive fixation, cardiac lead;

FIG. 2 is an enlarged side view showing the lead of FIG. 1 provided witha soluble covering in accordance with the present invention;

FIG. 3 is a view similar to that of FIG. 2, illustrating the covering atan intermediate stage of its formation;

FIG. 4 is a side view of a lead provided with a second embodimentcovering;

FIG. 5 is a side elevation of a lead provided with a third embodimentcovering comprising a molded tip;

FIGS. 6 and 7 are side views of leads provided with coverings comprisingpre-molded tips; and

FIGS. 8 and 9 are top and side views, respectively, of a tool used informing soluble coverings pursuant to the present invention.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT

Turning now to the drawings, there is shown in FIG. 1 the distal endregion of an implantable, positive fixation, cardiac lead 16. Devicessuch as lead 16 typically are inserted intravenously, for example intothe subclavian vein or the cephalic vein, and progressively moved towardthe heart until the distal end reaches a selected cardiac chamber. Withthe distal tip positioned at a selected location, the lead proximal end,still outside of the body, is maneuvered to implant the distal tip intothe endocardium. The implanted lead transmits electrical signals betweenthe selected location in the heart and the lead proximal end, for one orboth of two purposes: to monitor heart electrical activity at theselected location, and to carry stimulating signals to the selectedlocation from a pulse generator (not shown) connected to the leadproximal end.

To transmit the electrical signals there is provided an electricalconductor, shown in FIG. 1 as a double-wound coil 18 formed of a nickelalloy. The coil provides maximum flexibility for conforming to the vein,with minimal stress on the conductor. At the distal end of the lead isan electrode 20, electrically and mechanically coupled to coil 18 by aplatinum alloy crimp tube 22. A flexible, dielectric sheath 24 surroundsthe coil and crimp tube. A suitable material for the sheath is siliconerubber.

Electrode 20 is porous, having a screen 26 formed of a platinum alloy.Screen 26 aids in chronic fixation, whereby fibrous connective tissueintertwines with the screen to firmly secure electrode 20. Fibrousencapsulation, however, can take weeks, and it is essential to provide ameans for positively securing the lead distal end during the timeimmediately following implantation. To this end, there is provided afixation helix 28 of platinum alloy. Helix 28 has a sharp point at itsdistal end, which readily penetrates the endocardium. Upon initialpenetration, the helix is manipulated from the proximal end of lead 16,whereby it rotates clockwise, to further penetrate the tissue, to thepoint of firmly securing electrode 20 at the designated endocardiallocation.

A problem associated with helix 28 is that its sharp tip is capable ofsnagging and becoming entangled with the blood vessel wall, venousvalves, or heart valve. Consequently, the physician using lead 16typically is advised to rotate the helix counterclockwise, which tendsto draw the sharp point of the helix away from the tissue it encounters,to minimize the potential for entanglement. Alternatively, protectivedevices, such as those discussed above, have been employed.

FIG. 2 illustrates lead 16 with a covering or tip 30 mounted at itsdistal end in accordance with the present invention. Tip 30 is solid,and adheres to helix 28, screen 26 and the distal end of sheath 24. Theouter surface of tip 30 is generally spheroid. However, the precisesurface configuration is not so important as the fact that tip 30 issmooth, rounded and blunt, and that it completely surrounds fixationhelix 28 to protect intravascular and other tissue from the fixationhelix, particularly its sharp point 32.

Tip 30 is composed of a non-toxic, biocompatible and non-pyrogenicmaterial. Also, the material must be soluble in body fluids(particularly blood), within a temperature range encompassing normalbody temperature (37° C.). Further, the material of tip 30 must maintainits structural integrity in an environment of body fluids at or aboutnormal body temperature, in that it should not undergo plastic orelastic deformation as it dissolves. Usually the structural integrityrequirement is satisfied if the melting point of the tip issubstantially above normal body temperatures, in fact preferably 60° C.or higher.

In one example, tip 30 has been formed of mannitol, chemical formula C₆H₁₄ O₆. Mannitol has a melting point of about 167° C., and one gramdissolves in about 5.5 milliliters of water, with solubility beinggreater in hot water. Mannitol in a glass beaker was first heated to atemperature between 177° and 182° C., slightly above its melting point,and maintained at that temperature. The distal end of lead 16 wasimmersed in the mannitol solution for a brief time and withdrawn. Aportion of the mannitol adhered to helix 28, forming a core portion 34as illustrated in FIG. 3. Away from the beaker, helix 28 and coreportion 34 were allowed to cool a sufficient time for the core portionto solidify. This cooling required about five seconds.

Following cooling, the lead distal end was dipped into the mannitol meltonce again, then withdrawn after about one second. A second portion ofthe mannitol melt, adhering to core portion 34, helix 28 and screen 26,was sufficient in combination with the core portion to form tip 30 asillustrated in FIG. 2. The spheroid tip configuration results from thenatural surface tension of the mannitol melt as it solidifies. A tipformed in this manner dissolves in water heated to about 38° C. in aboutthree and one-half minutes.

Repeated trials of this example have yielded consistently satisfactorylead tips. The results indicate that the precise temperature of themannitol melt is not critical, so long as it is maintained in liquidform, slightly above the melting point. Likewise, the duration of eachdip coating is not critical, although it must be sufficient foradherence of subsequent mannitol layers while not so long as to meltmannitol previously solidified onto the lead. Finally, as the number ofdip coatings required is largely a function of desired tip size, certaintip designs may require substantially more than the two dip coatingsdescribed.

The dissolving time for tip 30 in body fluid is controlled principallyby the tip material, surface area and thickness so that increaseddissolving times can be provided, if desired, by increasing the tipthickness. The main concern is that tip 30 be of sufficient size toensure that lead 16 can be directed intravenously to the selectedcardiac chamber prior to exposure of helix 28, particularly at point 32.Also of concern is that within a reasonably short time after insertion,tip 30 is completely dissolved to expose helix 28 for the implanting ofelectrode 20.

While the above example involves mannitol, other sugar derivativesstable at temperatures below 60° C. are suitable substitutes, forexample dextrose, sorbose, sucrose, and glucosamine. Also usable arecertain salts, for example sodium chloride, potassium chloride andsodium carbonate. A further suitable constituent is polyvinylpyrrolidone(PVP). These materials are suitable, as well, with non-helical fixationelements.

The dip process described above, when used to form tip 30, tends to trapair which expands due to heating, and can cause undesirable formation ofbubbles in the tip. In such cases, it is advantageous to control thedegree to which the lead distal end is submerged into the constituentmelt. For example, FIG. 4 illustrates a lead 36 in which a tip 40entirely surrounds a fixation helix 42. Tip 40 does not abut a sheath44, but leaves a proximal portion 46 of a screen 48 exposed. As a resultof such controlled submersion, air can escape through proximal portion46, and later the proximal portion facilitates ethylene oxidesterilization of the lead distal end.

For improved control over the size and shape of the soluble tip, a tip50 for a lead 52 is formed by a casting or injection molding of aconstituent melt. The tip surrounds a helix 54 and screen 56, to abut asheath 58. As indicated at 60, the profile of the tip along its side islinear, while a rounded blunt distal end 62 is retained. This allows areduction of tip diameter to the nominal lead diameter, to furtherfacilitate intravenous insertion.

Another method of controlling the size and shape of the tip is topreform the tip by casting or injection molding. FIG. 6 illustrates alead 64 with a screen 65 and fixation helix 66 affixed to a conductorwhich is surrounded by a sheath 68. Surrounding the helix and screen isa preformed tip or capsule 70, fixed to sheath 68 using an adhesive at72. Such adhesive can be the molten material itself in the case ofmannitol, a heated syrup of fructose and sucrose which solidifies uponcooling, or syrups of other sugars (mannitol, sorbitol, etc.). A largediameter opening 74 is formed in capsule 70 to accommodate screen 65,while a cylindrical bore 76 of greater depth accommodates helix 66. Asindicated by broken lines at 77 and 78, bore 76 can be shaped to providea substantially uniform thickness in capsule 70 if desired. One or moreopenings, as indicated at 79, can be formed if desired to facilitatesterilization and increase the capsule surface area exposed to bodilyfluids during lead insertion.

Although it requires an adhesive not needed in the dip coating or directmold approach, a preformed tip has several significant advantages.First, it affords maximum control over the tip size and shape, so that acomparatively precise dissolving time can be achieved by appropriatelyselecting tip constituents and tip thickness. The resulting consistencyamong many tips renders the preformed tip the preferred choice for massproduction. Also, the preformed tip requires less of the tipconstituent. In particular, no constituent is provided where none isneeded--namely, in the cylinder defined by the fixation helix. Thisfactor contributes to more predictable dissolving times as well asmaterial savings. Preformed tips are more amenable to being installed orreplaced on site. Finally, constituents that cannot be melted because oftheir thermal instability, but which have the desired dissolvingproperties, can be formed into a tip such as tip 70 by compression ormolding. Examples of such constituents are lactose, lecithin incombination with other materials, and glucosamine.

In FIG. 7, an alternative preformed tip or capsule 80 is mounted to alead 82 over its fixation helix 84 and screen 86. A helical bore 88 isformed in tip 80, of a size and shape to accommodate helix 84, so thattip 80 is secured to lead 82 by turning it clockwise upon the helix. Somounted, tip 80 depends less upon an adhesive, and may not require anyadhesive to connect it to a sheath 90.

Shown in FIGS. 8 and 9 is a forming tool 92, which is an aluminum blockincluding two opposed sections 94 and 96 held together by socket headscrews 98 and 100. The opposing walls of sections 94 and 96 are cut awayto form cavities 102, 104 and 106 when sections 94 and 96 are mountedwith respect to each other as shown in the figures. Spacers, indicatedat 108, maintain a slight gap between sections 94 and 96, preferably ofabout 0.005 inches.

The cavities are substantially identical in shape, although they can beformed in different sizes corresponding to tips of different selectedsizes. Cavity 104, for example, includes a tip forming segment 110 andan upper chamfered segment 112. Tip forming segment 110 has the desiredcylindrical sides and rounded base to form the desired blunt tip, whilechamfered segment 112 facilitates insertion of tips prior to theirshaping, and also serves as a temporary catch basin for excess meltedtip material.

Forming tool 92 is used to control the size and shape of a soluble tipformed by the dip process described in connection with FIGS. 2-4. Moreparticularly, the lead distal end is dipped in the mannitol solution,permitted to cool, then dipped again, this process being repeated asufficient number of times to form a tip larger than the desired size.Then, after heating forming tool 92 to a desired temperature, preferablyslightly over the mannitol melting point, the tip formed by dip coatingis momentarily inserted into the desired one of cavities 102-106, thenquickly withdrawn, the desired time within the chosen cavity being afraction of a second. During this brief insertion, the heated cavitywall melts the excess mannitol, whereupon the melt is removed bydraining through the gap formed by spaces 108. Part of the excessmannitol melt is collected briefly in chamfered segment 112 beforedrainage. While not necessary, the tip can be rotated slightly about avertical axis while inserted, to further ensure the desired cylindricalsides and blunt end.

Whether formed by dip coating, direct molding or molded separately forlater attachment, a soluble tip in accordance with the present inventionrenders cardiac pacing lead implantation safer and less traumatic to thepatient. As it is moved toward the heart through a selected vein, thetip dissolves, but at a sufficiently slow rate to prevent exposure ofthe fixation helix or other fixation element, until the electrode is atleast near to its selected location along the endocardium. The smooth,rounded and blunt tip in fact expedites intravascular lead insertion.The tip thickness and constituent can be selected in accordance with theanticipated insertion time, with particular accuracy in the case of apreformed tip or capsule. Thus, within a brief period of time afterproper positioning, the fixation helix or other fixation element can bemanipulated from outside the body, in the usual manner, to secure theelectrode.

What is claimed is:
 1. An intravascular lead implantable inside apatient, including:an electrode having a fixation element for effectingpenetration into endocardial tissue at a selected location to securesaid electrode at said selected location; a flexible electricalconductor; a flexible, biocompatible dielectric sheath surrounding saidconductor along substantially the entire length thereof; and a couplingmeans for electrically and mechanically joining said electrode to adistal end of said conductor, whereby the conductor and electrodetransmit electrical signals between said selected location and aproximal end of said conductor; and a biocompatible covering surroundingsaid fixation element for facilitating intravascular movement of saidelectrode, said covering soluble in body fluids, stable in an ambientenvironment, having a melting point of at least 60° C., tending tomaintain its structural integrity in an environment of bodily fluids atapproximately body temperature, and having thickness selected to allowat least a predetermined minimum time for the intravascular insertion ofsaid electrode and the positioning of said electrode at least proximatesaid selected location, before said covering dissolves sufficiently toexpose said fixation element and permit said penetration, said coveringconsisting essentially of at least one of the following constituents:mannitol; dextrose; sorbose; sucrose; sodium chloride; potassiumchloride; and sodium carbonate.
 2. The intravascular lead of claim 1wherein:said covering consists essentially of mannitol.
 3. Theintravascular lead of claim 1 wherein:said covering consists of one ofsaid constituents.
 4. The intravascular lead of claim 1 wherein:saidcovering comprises a preformed capsule with means forming a bore in saidcapsule for accommodating said fixation element, and means joining saidcapsule with respect to said electrode with said fixation element insideof said bore.
 5. The intravascular lead of claim 4 wherein:said joiningmeans comprises an adhesive.
 6. The intravascular lead of claim 4wherein:said fixation element is a helix, and said bore has a helicalshape corresponding to the shape of said helix.
 7. The intravascularlead of claim 1 wherein:said electrode includes an electricallyconductive and porous electrode portion proximally of said fixationelement.
 8. The intravascular lead of claim 7 wherein:said coveringoverlies part of said porous electrode portion and leaves the remainderof said portion exposed.
 9. An apparatus for facilitating intravascularinsertion of a body implantable device, comprising:a body implantabledevice, a fixation element at a distal end of said device, and abicompatible, non-pyrogenic covering substantially surrounding saidfixation element, said covering having a melting point of at least 60°C., being stable in an ambient environment, soluble in body fluids,tending to maintain its structural integrity in an environment of bodilyfluids at approximatley body temperature, and having a thicknessselected to allow at least a predetermined time for the intravascularinsertion of said distal end at least proximate to a selected locationinside a body of a living animal, before said covering dissolvessufficiently to expose said fixation element to permit penetration ofthe fixation element into body tissue at said selected location, saidcovering consisting essentially of at least one of the followingconstituents: mannitol; dextrose; sorbose; asucrose; sodium chloride;sodium carbonate; and potassium chloride.
 10. The apparatus of claim 9wherein:said covering consists essentially of mannitol.
 11. Theapparatus of claim 9 wherein:said covering consists essentially of oneof said constituents.
 12. The apparatus of claim 9 wherein:said bodyimplantable device comprises a cardiac pacing electrode.
 13. Theapparatus of claim 12 wherein:said covering is formed by dip coating amaterial comprising said covering in liquid melt form onto said fixationelement for solidifying in surrounding relation to said element.
 14. Theapparatus of claim 12 wherein:said covering comprises a preformedcapsule and means forming a bore in said capsule to accommodate saidfixation element, said apparatus further including means for connectingsaid capsule with respect to said electrode with said fixation elementcontained in said bore.
 15. The apparatus of claim 14 wherein:saidfixation element is a helix, and said bore has a helical shapecorresponding to the shape of said helix.
 16. The apparatus of claim 14wherein:said electrode includes a porous screen, and said capsule, whenconnected with respect to said electrode, overlies most of said screen,leaving the remainder of said screen exposed.
 17. The apparatus of claim12 wherein:said electrode includes an electrically conductive and porousscreen located proximally of said fixation element, and said electrodeis selectively dip coated to cover said fixation element and a portionof said screen, leaving the remainder of said screen exposed.
 18. Anintravascular lead implantable inside a patient, including:an electrodehaving a fixation element for effecting penetration into endocardialtissue at a selected location to secure said electrode at said selectedlocation; a flexible electrical conductor; a flexible, biocompatibledielectric sheath surrounding said conductor along substantially theentire length thereof; and a coupling means for electrically andmechanically joining said electrode to a distal end of said conductor,whereby the conductor and electrode transmit electrical signals betweensaid selected location and a proximal end of said conductor; and abiocompatible covering surrounding said fixation element forfacilitating intravascular movement of said electrode, said coveringsoluble in body fluids, stable in an ambient environment, having amelting point of at least 60° C., tending to maintain its structuralintegrity in an environment of bodily fluids at approximately bodytemperature, and having a thickness selected to allow at least apredetermined minimum time for the intravascular insertion of saidelectrode and the positioning of said electrode at least proximate saidselected location, before said covering dissolves sufficiently to exposesaid fixation element and permit said penetration, wherein said coveringis formed by dip coating said fixation element with a liquid melt ofmaterial comprising said covering.